Before "Fault in Our Stars," the ABC Family smash — now hitting its 100th episode — had proved that adolescents still turn out en masse for the right entertainment (#socialmediahelpstoo).

On March 18, Pretty LittleLiars brought Twitter to a fevered pitch when in the final minutes of the season-four finale, the villain known as "A" nearly was exposed by a beloved character, Ezra, who found himself taking a bullet to protect the main characters. His unclear fate was magnified by the worldwide-trending #IsEzraAlive hashtag and drove PLL's highest tweet volume for the year with 1.45 million. The finale is the top tweeted television episode on Twitter to date for 2014.

Fueled by an enviable online presence — 13.7 million Facebook fans, 2.22 million on Twitter and 668,000 Instagram followers — it's no wonder PLL, whose median viewer age is 21, has become the "how-to" in keeping an elusive demographic, younger viewers, deeply engaged. While live viewership among the younger set has been declining in favor of gaming consoles and mobile devices, according to a 2013 Nielsen cross-platform report, PLL has bucked the trend, pulling in night-of viewing with steadily increasing ratings. (Season four averaged 2.7 million viewers in time-shifted viewing among the target 12-to-34 demo, up from 2.5 million in season two.)

"In an environment where TV has become fragmented, the show has become stable, and they're watching it live," says LeslieMorgenstein, CEO of Alloy Entertainment. Warner Bros. TV Group president Peter Roth says the property compares well against the rest of the studio's portfolio: "Pretty Little Liars has resonated about as much as anything we've ever done." Adds network president TomAscheim, who on June 10 secured ABC Family's No. 1 series with a rare two-season pickup, "This is our version of the Super Bowl."

So how does the creative team do it? By allowing its viewers a significant place in the PLL universe, driven by a social footprint that is disproportionately large compared to its traditional ratings (averaging 3.9 million total viewers in live-plus-7). It is the third-most-tweeted-about 2013-14 series after Breaking Bad and The Walking Dead, with an average reach of 4.8 million users, trumping Game of Thrones (3.5 million users vs. 9.3 million viewers to the June 15 season finale). Life is extended beyond the TV screen by continuing the conversation year-round, through plot-specific hashtags, Facebook apps and targeted use of Twitter and Instagram. The social media space is where the show's core audience lives: 31 percent of Twitter users fall under 30, compared with only 19 percent for adults 30-to-49, according to Pew Research.

"Pretty Little Liars does a good job of meeting fans where they are," says Jenn Deering Davis, co-founder of Union Metrics. "They were one of the first shows to spend significant investment on social media." Even before PLL debuted, a Twitter account was created in 2010, "the same time Twitter found its legs," says actress TroianBellisario, 28. "We don't have watercooler talk the next day; we have watercooler talk the minute it happens as you're watching the episode." Danielle Mullin, vp marketing at ABC Family — who now oversees a dedicated team of six to 10 live tweeters per episode, including series stars — says, "We were very surprised. The initial reaction on Facebook was a sign of this incredible hit on our hands."

Based on Sara Shepard'sYA novels, PLL revolves around four aspirationally dressed teen girls — Aria, Hanna, Emily and Spencer — living in Rosewood, Pa., who are stalked by a faceless "A" after their friend, Alison, mysteriously disappears. Developed at Warner Bros.-owned book-packaging/production company Alloy in 2004, the premise came out of a desire to launch a teen equivalent to the biggest soap at the time, ABC's Desperate Housewives.

PLL's TV success extended the life of Shepard's best-selling novels from eight to 16 books, the final one hitting shelves in December, and created ancillary product lines. In February, the show spun off the network-mandated edgy-yet-affordable wardrobe in an Aeropostale partnership with costumer Mandi Line (a summer collection launched in June). The show also features musical acts that reflect its moody DNA, such as MS MR, Lily Lane and ZZ Ward. And before the show's 2010 premiere, The Pierces' 2007 "Secret," PLL's theme, had sold 53,000 downloads from U.S. retailers; afterward, sales nearly tripled to 129,000, per Nielsen SoundScan.

Led by Lucy Hale, 25; Bellisario; Ashley Benson, 24; Shay Mitchell, 27; and Sasha Pieterse, 18, who have more than 11.3 million Twitter followers combined, the series — the first Warner Horizon scripted production and second ABC Family show to hit 100 episodes — made a splash in 2010 for the network, which had ShaileneWoodley's teen pregnancy drama The Secret Life of the American Teenager and was itching for an identity jolt. Recalls Kate Juergens, ABC Family chief creative officer, about former president Paul Lee'sgreenlighting the show: "It allowed us to get sexy in a way we hadn't before and it opened the door to a more sophisticated kind of programming." She also credits an aggressive marketing approach by Lee, now ABC's entertainment chief — "he was committed to being big, bold and flashy with the show. He took no prisoners and wanted it everywhere." PLL was a world previously unseen on ABC Family. "Everything — down to the characters, the wardrobe, the dialogue, the tone — was so specific. They had a vision and that's why it completely worked," saysHale, who plays Aria and was the first to be cast. "We sort of stick to our guns on what our show is. We haven't strayed from what made our show so successful in the beginning. We haven't tried to fit a certain mold."

To keep storylines accelerated and the twists at "OMG!" level for an audience anxious to dissect every morsel online, executive producer I. MarleneKing says "one of the early mandates [was]: 'Don't write these girls as teenagers, let's write them as people.' " Another early note, executive producer Oliver Goldstick says, was to create a heightened reality that still felt attainable: "The one thing I know that ABC Family felt strongly about, which I think helped us, was, 'Please don't make this a rarified world. Don't make them wealthy kids.' Making sure it was an idealized Middle America helped." For executive producer JosephDougherty (thirty­something, Judging Amy), there was no precedent before PLL for intensely serialized storytelling. "A feud on thirtysomething might last half a season," he says. "When Aria and Hanna were at odds, the network said, 'We'd like to accelerate it and let them get back to being friends.' The volatility of adolescent characters lets you ... structure the story in a way that lends to crescendos."

PLL also isn't afraid of diving into more adult, sometimes difficult, subject matter. Various storylines have focused on sexuality, depression, substance abuse and a student-teacher romance, just to name a few. Says Hale, "I think a lot of teen shows shy away from risky topics and risky issues. I know when I was a teenager, I would have loved a show like this because everything is a little risque." The "hostile atmosphere" of high school, Goldstick says, is a prime setting for a show that "at the end of the day is about fear of exposure. Life does not change that much. Our loves are still an extension of that time."

Milestone moments on PLL informed ABC Family's social stunts and supplemental offerings. Digital spinoff series Pretty Dirty Secrets, which posted 3.6 million views during a season-three break, introduced a new character, Shana. "A lot of other shows stop tweeting and don't start again until a month before the season premiere. Pretty Little Liars keeps going and that keeps viewer interest," says Davis. With an understanding of how young adults use social media platforms, PLL leans on Twitter for real-time updates, Instagram for behind-the-scenes photos and Tumblr for more fan-created content, like GIFs. "The real value is letting fans feel like they're insiders. It's not just tweeting at them, it's retweeting, replying, getting talent to tweet, giving special access," notes Davis.

"We want to give value for that Facebook like or Twitter follow," adds Mullin, who calls the strategy that feeds fan speculation with preview videos, cast messages and teases "responsive marketing." Davis says PLL's tactics are "sophisticated. ABC Family [asks], 'When do we get the highest tweets per minute and at what point in the episode? How can we nurture that?' "

Nearly the entire PLL cast and writing staff are on Twitter, something Bellisario, who plays Spencer, had trouble grasping at first: "It was kind of required by the network. Because we were so popular on Twitter, it [became] my choice to be on it because when I wasn't engaging, there were a lot of people pretending to be me, which irked me." How they choose to engage and how often is up to them, though Dougherty says Twitter has become slightly "managed." ABC Family provides "a digital asset package" before a new episode "with suggestions for what they might like to post along with photos and clips should they want to use it," says Mullin. The key is knowing how much information to dole out. "What we've learned is to keep the audience engaged by answering the questions that we can, that don't invade the personal lives of the actors or crew, and try not to give anything away," says Dougherty.

It wasn't until the stars went abroad that they fully grasped the show's global reach. PLL airs in 28 territories and is the No. 1 drama in its time period in countries like Italy, says Warner Bros. TV Group chief marketing officer Lisa Gregorian. "It's like, ‘Wow, our show is world-renowned,' " says Pieterse, who plays Alison, of PLL's international success. Mitchell, who plays Emily, recounted a Brazil trip with Hale ahead of the second season where the pair "couldn't leave our hotel rooms without security. Then I realized it wasn't just Brazil, it was all over."

In the end, the show's success comes down to intriguing storytelling. "PLL is a perfect mix of romance, drama and suspense that really connects with our viewers around the world," says Mullin. "When you have those ingredients, the Internet and social media provide a fantastic forum for amplifying the buzz and pushing conversation viral."

5 Record-Breaking Social Media Stunts

"A Is Everywhere"

ABC Family execs noticed viewers posting photos of "A"s from their lives and launched an "A Is Everywhere" contest in 2011, with a set visit as a prize; 753,554 fans posted on the Facebook app, generating 2.32 million views and 4,975 submissions. A poster of the top 50 images was created (see right).

#ADay

The #ADay hashtag was assigned for the March 2012 season-two finale "unmAsked," which revealed Mona as (the first) "A," or bully. It set a Twitter record for a TV episode, peaking at 32,000 tweets a minute and posting 645,000 for the hour, according to SocialGuide.

#TheBetrAyal

Three weeks before the season-three summer finale in August 2012, the network launched a countdown campaign for a bombshell betrayal. An interactive suspect tracker Facebook app allowed viewers to vote on the betrayer's identity (Toby). It generated more than 709,000 tweets, peaking at 36,000 a minute.

#WorldWarA

An on-air promo three weeks before the mid-season-four finale in August 2013, "Now You See Me, Now You Don't," hinted at a reveal of an unlikely "A" (Ezra). Knowing viewers shared photos and videos of reactions to past reveals, the network compiled a digital mosaic of Ezra on Facebook after the show aired. One day post-reveal, "Pretty Little Liars" and "Ezra Is A" still were trending on Twitter.

#PLLonGMA

On the stars' March Good Morning America appearance to tout the season-four finale, co-host Robin Roberts provided half of a code word. A promo then aired during the finale, "A Is for Answers," to tease the second part, with the full clue leading to a website with a message from King — played more than 400,000 times — hinting at season five.